1. The Field of the Invention
This invention relates to devices and methods for discriminating between different types of electrical signals and, more particularly, to novel apparatus and methods for electronically distinguishing between voice signals and system-generated signals in an electrical audio device.
2. The Prior Art
During recent years, largely as a result of the significant developments in the microprocessor arts, a number of devices have been developed which are designed to call one or more specified telephone numbers and then transmit and/or receive an audio message. For example, security devices have been developed which will, upon being actuated, call one or more designated telephone numbers and deliver an emergency message and/or a request for help. Similar devices are now being used by many doctors, dentists, and other professionals to call patients or clients and remind them of scheduled appointments. Many department stores are likewise using such devices to call customers and inform them that their has order has been received. Also, marketing companies and salesmen have begun using automatic calling devices to contact potential clients or customers, to inform them of a new product or service, and/or to obtain answers to particular questions. In fact, there are presently numerous types of such microprocessor-based devices which are being used in various ways to perform similar automatic telephone calling functions.
When using any such automatic calling device, it is, of course, intended that the audio message will actually be heard by an individual who answers the called telephone. Nevertheless, one of the principal challenges associated with these devices has been to develop a dependable way of determining that a party has, in fact, answered the called telephone.
The design of early automatic calling devices often simply ignored the problem of detecting whether the called telephone had been answered. Thus, some of these early devices would simply dial the designated telephone number and then deliver the message. It will, of course, be appreciated that the called party would often not receive the audio message at all, such as, for example, if the party was not home or if the telephone line was busy. In other cases, the called party would receive only part of the message, since the audio message would have been started before the called party ever answered the telephone.
In an effort to overcome these problems, some of the early automatic calling devices were designed so as to repeat the audio message a specified number of times after dialing the telephone number and before disconnecting. Thus, for example, the device might dial a telephone number and then repeat the audio message two times. As a result, a party might answer the telephone in the middle of the first time through the message, but would still be able to receive the entire message before the device was disconnected. Understandably, however, this method of message delivery could be rather annoying to call recipients. More significantly, repeating the audio message several times still did not ensure that the called party ever received the message at all, such as, for example, if the telephone line was busy or if the called party did not answer.
Recognizing the need for some type of telephone answer detection, a number of attempts have been made by those skilled in the art to incorporate into automatic calling devices some way of detecting that the called telephone has been answered. In this way, it was hoped that the device could then be designed so as to deliver the audio message only after the called telephone was answered, thereby avoiding the problems outlined above. However, the devices 22 and methods which have heretofore been used to detect whether a called telephone has been answered have proven to be largely inadequate and undependable.
One method which has previously been used to determine whether a called telephone has been answered is to require the answering party to transmit a recognizable answer signal back to the calling device. For example, the calling device could be designed to dial a designated telephone number and then wait to receive a specific tone or other answer signal before transmitting the audio message. Such an answer signal could, for example, be a conventional dual-tone multifrequency signal which is transmitted manually from the telephone of the answering party. Alternatively, the answer signal could comprise a tone or other signal which is transmitted automatically by a second device that is connected in some way to the called telephone.
While this method of telephone answer detection may be quite reliable, it has some significant disadvantages. First, since telephone companies do not provide any universal telephone answer signal, the calling device cannot generally be used to call a particular party until prior arrangements have been made with that party. For example, this answer detection method typically requires either that the called party first know the required answer signal to transmit or that a piece of special equipment be previously installed in connection with the party's telephone. As a result, this method of detecting that a called telephone has been answered has proven to be quite cumbersome and inconvenient and may also be relatively expensive. Moreover, due to the required prior arrangements, this method of telephone answer detection may even preclude the use of the automatic calling device for many of the purposes outlined above.
As alluded to previously, in many areas throughout the United States, the telephone company provides an electronic signal indicating that a telephone has been answered. This signal generally comprises reversing the voltage polarity on the telephone lines (that is, causing the electrical current to flow through the telephone lines in an opposite direction after the telephone has been answered). A reversal in voltage polarity is relatively easy to detect, and such detection can be done with a great degree of reliability. Hence, some prior art devices have attempted to detect an answered telephone by simply detecting a reversal of voltage polarity on the telephone lines.
The major impediment to this method of detecting an answered telephone is that the telephone company in many parts of the United States, as well as in foreign countries, does not always provide voltage polarity reversal when a telephone is answered. As a result, this method of detecting an answered telephone, while simple and quite reliable, cannot be used uniformly on all telephone systems. Thus, if a telephone call is placed into an area in which voltage polarity reversal is not provided as an answer signal, no answer will be detected by the calling device.
Still other prior art devices have attempted to detect the answering of a called telephone by monitoring the telephone system's call progress tones. In general, there are only four different types of call progress tones to be monitored: (1) a dial tone, (2) a ringing tone, (3) a line-busy signal (slow busy signal), and (4) an all trunk busy signal (fast busy signal). By monitoring the call progress tones, for example, it can be determined whether the called telephone has started ringing. If the telephone thereafter stops ringing, it may be presumed that the telephone was answered.
Call progress tones have previously been monitored by distinguishing between the timing of the audio signals associated with each tone. For example, a ringing tone may comprise a two-second audio tone followed by four seconds of silence, with this pattern then repeating. Thus, if this audio signal pattern were detected on the telephone lines, the calling device would assume that the called telephone is now ringing.
Again, one of the chief problems associated with this method of detecting if a called telephone has been answered is due to the nonuniformity between various central offices of the telephone sytem. For example, in some areas, a ringing tone may comprise a two-second tone followed by only three seconds of silence, while other areas may maintain the pattern comprising four seconds of silence, as set forth above. As a result, it may not be possible to adequately recognize and/or distinguish between various call progress tones; and this nonuniformity in timing thus makes this method unreliable for universal use in detecting whether a telephone has been answered.
In addition, the above-described method of detecting an answered telephone may not respond quickly enough to the called party. For example, the party may answer the telephone in the middle of a ringing tone and say "hello." The party may then wait four seconds before saying "hello" again, and the calling device would generally perceive this as a continuation of the ringing tone. In all likelihood, the called party would soon hang up the telephone, and the calling device would not have yet delivered the audio message.